The selection criteria used to obtain the data
from the USGS website
on earthquakes was all earthquakes within a 700 km radius
of a point at 90 longitude and 40 degrees latitude, which is centered
just a bit north of the Tibetan plateau.

After computing the number of earthquakes >
certain size seen per year in the 27 years of record the plots
below were generated in Excel. The steps were to first rank the
earthquakes by sorting in descending order and assigning an associated
rank value in the next column. Then dividing the rank by 27 gives
the number of earthquakes that size or larger per year. Considering
the area, then this gives the number of earthquakes per year for
that area, sort of a frequency density. The log was taken of this
yearly frequency, and plotted as y against earthquake magnitude
(which is already a log scale) as x. All the data yielded the
plot to the left. An r-squared value of .91 suggests a decent,
although not great, fit.

It is fairly common in these plots to see a
curve at the lower magnitude end of the scatter plot, as you do
here. This is a resolution screen operating on the data. Earthquakes
in a certain size range can not be consistently measured by the
seismic network, and below that they are invisible to the seismic
network. By looking at the data one can estimate that below a
magnitude 4 the network is not detecting all earthquakes in the
region. If we eliminate those earthquakes we get a more useful
linear regression with a higher correlation coefficient (r-squared=.96).
Notice, however, that we must have good reason for removing our
data from the analysis, and that we must make it clear that it
was omitted. A careful look may suggest that we have not quite
removed all of it.

While there is a good fit, note that at the
very stretch of greater interest, that of larger earthquakes,
we see the largest deviations from the line, and it is in a consistent
manner. Considering the scale and extrapolating one might estimate
a Richeter magnitude 8 earthquake should occur every hundred years
or so. This is somewhat consistent with the tectonic setting.
Note that this analysis does not speak to the scale limits on
the fractal distribution of earthquakes. Put another way, we haven't
addressed the important questions, what is the largest earthquake
possible or likely in this area. More analysis and caution is
needed if one wanted to speculate on the chances of a larger earthquake,
but this shows a way to start. For estimating the frequency of
moderate size earthquakes in the area this relationship would
be far more dependable.